Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer

Abstract

Turbulent fluxes of chemically reactive trace gases in the neutral atmospheric boundary layer (ABL) were simulated with a one-dimensional, coupled diffusion-chemistry model. The effects of rapid chemical reactions were included with a suite of second-order turbulence equations in which additional chemical terms were used to describe contributions to flux by rapid chemical production and loss. A total of 69 chemical reactions were incorporated to describe basic atmospheric photochemistry coupled with chemistry for isoprene and its oxidation products. Daytime flux Profiles of O3, NO, N02, OH, isoprene, and other depositing gases were simulated with assumed rates of NO emission from soil, isoprene emission rates appropriate for a deciduous forest, and initial concentrations of various chemical species typical of a remote area. Results show that chemical reactions can influence vertical fluxes by producing sources or sinks in the atmosphere and by changing mean concentrations. Magnitudes of NO and NO2 fluxes decrease with height at a much greater rate than predicted by a nonreactive model. The NO emitted from soil can quickly be converted to N02, and the upward NO flux can decrease by as much as 80% at a height of 100 m. The magnitude of NO2 flux decreases sharply with height because of the NO-to-NO2 conversion, but NO2 deposition near the surface tends to be enhanced by an increase in NO2 concentration near the surface NO emission source. The Profile of 03 flux simulated with forced entrainment at the top of the ABL closely matches the profile derived from a field experiment, and the flux throughout the ABL increases slightly because mean O3 concentrations are increased by chemical production associated with isoprene emissions. Simulated profiles of isoprene flux closely agree with results of a nonreactive model and appear to be controlled primarily by surface emission and vertical turbulent mixing. Chemical reactions appear to have a substantial effect on vertical concentration gradients, diffusivities, and deposition velocities for NO2, NO3, and N2O5. The reactions have a negligible effect on the deposition velocities for O3, HCHO, CH3OOH, HNO2, H2O2, and HNO3.